A nuclear power plant in general is configured to generate electric power in such way that a first cooling water is heated using the energy generated during a nuclear fission which is carried out using nuclear fuels in the inside of the nuclear power plant, and a steam turbine is rotated using the steam generated by a steam generator after the energy is transferred to a second cooling water with the aid of the energy of the heated first cooling water, and the rotational kinetic energy of the steam turbine is converted into an electric energy by an electric power generator. Inside the nuclear power plant, a number of pipes with various sizes are connected so as to supply a first line steam heated inside a nuclear reactor and various cooling waters, and the pipes are connected by a welding method.
Among the aforementioned various size pipes installed inside the nuclear power plant, small bore pipes are installed, the diameters of which are smaller than or equal to 50 mm. A socket may be connected between the pipes so as to change the directions of the small bore pipes, and the pipe and the socket are connected by the welding method.
The welded portions of the small bore pipes may continuously receive fatigue-associated damages due to the mechanical vibrations which are transferred from various devices during the operation of the nuclear power plant. For this reason, errors may frequently occur. In order to prevent the aforementioned errors, it needs to check any defects at the welded portions of the socket with the aid of an ultrasonic inspection method during the operation of the nuclear power plant.
The ultrasonic inspection method currently used for the small bore pipes inside the atom power plant in general is carried out in such a way that an inspector supplies an ultrasonic wave toward the welded portions of the socket from the side of the pipe or a coupler by using an ultrasonic probe, and an ultrasonic wave which has reflected off any defect portion is received, thus evaluating a defect state based on the signals which have shown at an ultrasonic inspection meter.
According to the aforementioned manual inspection method, an inspector should check the signals while moving with the user's hand holding the ultrasonic probe, whereupon it is difficult to maintain the contacts constant between the probe and the pipe, and the signals at the screen of the ultrasonic inspection meter should be analyzed while simultaneously checking the contact state between the ultrasonic probe and the pipe. In this case, the inspector cannot focus on the signal evaluation work, so the defect-associated signals may be missed.
A scanner has been newly developed so as to improve the aforementioned problems, wherein the scanner can be rotated while maintaining a predetermined constant contact state between the ultrasonic probe and the pipe.
The Korean patent registration number 10-1103801 (a scanner for inspecting the volume of a narrow space fillet welding portion which was registered on Jan. 2, 2012) describes a scanner which is able to detect any defect at a pipe welding portion in a narrow space.
The scanner described in the Korean patent registration number 10-1103801 is coupled to a pipe using an upper case and a lower case, and a plurality of driving wheels are provided at the insides of the upper case and the lower case. The scanner can rotate in the circumferential direction of the pipe in a state where the upper case and the lower case are coupled to each other.
The welded portions can be inspected while moving in the longitudinal direction of the pipe the probe fixed at a probe holder which is provided at an end portion of the index the length of which can be adjusted per unit (refer to the paragraph number [0035] of the Korean patent registration number 10-1103801) of one step at the top of the upper case.
In case of the aforementioned Korean patent registration number 10-1103801, an insertion groove into which the pipe is inserted is formed at a contact portion between the upper case and the lower case, and the driving wheels are disposed with an angle of 120° between them in such a way that the rotation center can position at the same distance with respect to the center of the insertion groove, which makes it possible to respond to any change in the diameter of the pipe.
More specifically, the small bore pipes have the diameters between 10 and 50 mm. In case of the aforementioned registered patent, since it is impossible to flexibly respond to any difference in the diameter of the pipe, the upper case and the lower case should be additionally manufactured and supplied, which are able to respond to the sizes of each pipe.
When it needs to inspect the pipes with various diameters, the inspector should carry a plurality of scanners which can be coupled to the pipes with different diameters, thus causing inconveniences, and the manufacturing thereof costs a lot.